It is well known in the art of electrochemical cells to provide a cell having a cathode of divalent silver oxide (AgO). When associated with a suitable anode, e.g., zinc and a suitable electrolyte, e.g., aqueous sodium hydroxide containing or saturated with zinc oxide, this material provides a cell of relatively high coulometric capacity per unit volume, and desired voltage.
The art is also aware that AgO, by itself, has certain deficiencies as a cathode material. For example, in storage and between periods of use, AgO in contact with cell electrolyte can decompose, releasing gaseous oxygen, thus simultaneously increasing pressure inside a sealed cell and reducing coulometric capacity. Furthermore, AgO provides dual voltage when electrolytically discharged. Initially upon discharge, AgO provides an undesirably high voltage characteristic of the AgO Ag.sub.2 O reaction and thereafter provides a lower, desirable voltage characteristic of the Ag.sub.2 O Ag reaction. Still further, a cell employing unmodified AgO as a cathode may be characterized by a changing internal resistance, namely an internal resistance which increases with time, both at room temperature and at elevated temperatures. Heretofore, numerous disclosures have been made which suggest methods of treating one or more of these problems, including:
______________________________________ Country Patent No. Inventor/Applicant Issued ______________________________________ Japan 78/63542 Murakami et al 1978 USA 3,017,448 Cahan 1962 USA 4,015,056 Megahed et al 1977 USA 4,078,127 Megahed et al 1978 USA 4,835,077 Megahed et al 1989 ______________________________________
The Murakami et al disclosure appears to be similar to that of Cahan in U.S. Pat No. 3,017,448. Both Cahan and Murakami et al disclose that the gassing of AgO can be inhibited by treating AgO with lead or lead component. Cahan discloses Pb, PbO, Pb(OH).sub.2 and sodium plumbate as materials suitable for addition to an AgO electrode or treatment of an AgO electrode, while Murakami et al teaches treating AgO with an alkaline solution containing a plumbate ion. Cahan specifically discloses that use of his treatment enhances or extends the duration of cell discharge at the undesirably high voltage characteristic of the AgO Ag.sub.2 O reaction. Thus, even though Cahan tended to solve the gassing problem associated with use of AgO cathodes, the solution exacerbated the afore-stated dual voltage problem.
Megahed et al, in U.S. Pat. No. 4,078,127 ("Megahed I") claims the use of metallic sulfide additives, incorporated into the AgO cathode material by dry mixing metallic sulfides and AgO. Specifically, Megahed I teaches that sulfides of cadmium, calcium, mercury, tin, tungsten, (or mixtures thereof) included in the cathode mixture, inhibit AgO gassing. Megahed I discloses that some of the sulfides, when included in the cathode mix, reduce cell impedance both initially and during discharge. Further, Megahed I discloses that cadmium sulfide is effective as a sulfide additive under the conditions of mix preparation prescribed therein. However, under the conditions so prescribed, Megahed I discloses that AgO cathode mixes which contain up to 3% lead sulfide are not beneficial to the stability of such AgO electrochemical cells, since such cathode mixes were detrimental to the impedance of cells upon high temperature storage. Moreover, Megahed I, while stating "cadmium sulfide was found to improve the voltage stability of cells . . . ", does not disclose, much less claim, that cadmium sulfide additives yield cells with a single voltage discharge under low drain conditions.
Megahed et al, in U.S. Pat. No. 4,015,056 ("Megahed II"), herein incorporated by reference, discloses and claims a two-step process which yields an AgO cathodic material having a single voltage discharge. In the first step, an AgO pellet is treated with a mild reducing agent, such as methanol to form a thin outer layer of Ag.sub.2 O on the surface of the pellet. Then, after consolidation, the pellet is treated with a strong reducing agent such as hydrazine, to form a silver layer on the pellet surface which faces the typical internal plastic membrane. Cells containing such cathodes display the OCV and CCV values typical of the Ag.sub.2 O--Zn electrochemical cell system, apparently controlled by the thin outer layer comprising monovalent silver oxide and/or elemental silver. However, with time, such cells develop a relatively high internal impedance and high open circuit voltage (OCV).
U.S. Pat. No. 4,835,077 to Megahed et al ("Megahed III"), herein incorporated by reference, suggests reacting divalent silver oxide with powdered lead sulfide in hot alkaline solution; whereby the resulting particles are taught to have an interior comprising AgO, a thin outer layer comprising silver and lead, and a thin intermediate layer of Ag.sub.2 O. It appears that the two outer layers are formed concurrently during the reaction process carried out in hot alkaline reaction medium. The resulting cell appears to have the desired voltage characteristic of monovalent silver oxide, while having a constant low internal impedance.
In addition to the above listed references, there are other references pertaining to the properties, formation and stability of AgO. "Electrode Phenomena of Silver-Silver Oxide System in Alkaline Batteries" by Yoshizawa and Takehara published in the Journal of the Electrochemical Society of Japan, Volume 31, Number 3, pages 91-94 (1963) reports the effect of various metallic additives on the oxidation of silver electrodes. Among the additives suggested, gold was reported to increase the rate of formation of divalent silver oxide during the electrochemical formation of silver electrodes, e.g. oxidation of silver.
Another article entitled "The Electric Resistivity of Silver oxide" by Tvarusko published in the Journal of the Electrochemical Society, Volume 115, Number 11, pages 1105-1110 (November, 1968) reported on various metallic additives and their effect on the electric resistivity of divalent silver oxide. The article reports that mercury and lead added during the preparation of silver oxide decreased the electric resistivity of silver oxide. The article further confirms the disclosure of Cahan in U.S. Pat. No. 3,017,448, that using lead in an AgO cathode extends the time during discharge at which the cell exhibits the undesirably high voltage characteristic of the AgO Ag.sub.2 O reaction.
U.S. Pat. No. 3,617,384 to Kamai et al discloses a secondary zinc alkaline cell in which gold or silver, their alloys, oxides and hydroxides, may be added to a zinc anode.
U.S. Pat. No. 3,650,832 to Tvarusko discloses additives for divalent silver oxide selected from mercury, selenium, tellurium, and combinations of mercury, with tin or lead.
Japan Patent Application No. 48-1929, opened in 1973, discloses adding gold hydroxide to the electrolyte of an alkaline silver oxide cell.
U.S. Pat. No. 3,853,623 to Davis discloses gold ion additive for divalent silver oxide cathode material.
U.S. Pat. No. 3,936,026 to Howard discloses, as cathode materials, mixtures of divalent silver oxide with (i) sulfur and (ii) silver sulfide.
British Patent 1,065,059 discloses the addition of cadmium oxide, magnesium oxide, or aluminum oxide to a silver electrode.
While Megahed III does provide a stable, single voltage, AgO cathode material having low impedance throughout discharge, by incorporating lead additives into the cathode material, the toxicity and ultimate disposal of the lead compounds used therein are of concern.
It is an object of this invention to provide an AgO cathode material similar to that of Megahed III, and producing equivalent electrochemical properties, but without the toxicity concerns which attend the use of lead.
It is another object to provide such an AgO cathode material having low impedance.
Still another object is to provide a low-toxicity AgO cathode material which can be used to make an electrochemical cell which is thermally stable, has low internal impedance, and displays a single voltage discharge at even low drains, over at least 95% of the cell discharge capacity.
Yet another object is to provide an alkaline electrochemical cell having low toxicity, substantially single voltage discharge, and low impedance.
Another object is to provide a method of producing a cathode material wherein the cathode material is a reaction product of divalent silver oxide and a bismuth compound, and wherein the reaction product comprises a compound of silver, bismuth, and oxygen.